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Carbonyl Sulfide Hydrolase from <i>Thiobacillus thioparus</i> Strain THI115 Is One of the β‑Carbonic Anhydrase Family Enzymes
Carbonyl sulfide
(COS) is an atmospheric trace gas leading to sulfate
aerosol formation, thereby participating in the global radiation balance
and ozone chemistry, but its biological sinks are not well understood. <i>Thiobacillus thioparus</i> strain THI115 can grow on thiocyanate
(SCN<sup>–</sup>) as its sole energy source. Previously, we
showed that SCN<sup>–</sup> is first converted to COS by thiocyanate
hydrolase in <i>T. thioparus</i> strain THI115. In the present
work, we purified, characterized, and determined the crystal structure
of carbonyl sulfide hydrolase (COSase), which is responsible for the
degradation of COS to H<sub>2</sub>S and CO<sub>2</sub>, the second
step of SCN<sup>–</sup> assimilation. COSase is a homotetramer
composed of a 23.4 kDa subunit containing a zinc ion in its catalytic
site. The amino acid sequence of COSase is homologous to the β-class
carbonic anhydrases (β-CAs). Although the crystal structure
including the catalytic site resembles those of the β-CAs, CO<sub>2</sub> hydration activity of COSase is negligible compared to those
of the β-CAs. The α5 helix and the extra loop (Gly150–Pro158)
near the N-terminus of the α6 helix narrow the substrate pathway,
which could be responsible for the substrate specificity. The <i>k</i><sub>cat</sub>/<i>K</i><sub>m</sub> value, 9.6
× 10<sup>5</sup> s<sup>–1</sup> M<sup>–1</sup>,
is comparable to those of the β-CAs. COSase hydrolyzes COS over
a wide concentration range, including the ambient level, <i>in
vitro</i> and <i>in vivo</i>. COSase and its structurally
related enzymes are distributed in the clade D in the phylogenetic
tree of β-CAs, suggesting that COSase and its related enzymes
are one of the catalysts responsible for the global sink of COS